This invention relates generally to the field of cement, especially cement for underwater application.
Cement is used underwater for many purposes including, for example, in pools, dams, piers, retaining walls and tunnels. Cement can be cast and repaired out-of-water and subsequently submerged, but this method is often undesirable or impossible. For example, draining water out of a filled swimming pool to repair it is undesirable as this can damage the pool and waste large amounts of valuable water. The damage occurs when the equilibrium between the water weight pushing downward and the ground and pool pushing upward is unbalanced, causing the walls and bottom to crack and separate. Similarly, removing water from a dam to repair it is undesirable because of the resulting structural compromises and water waste. It is often impracticable to remove the water surrounding submerged pylons of piers to repair them. When the cement structure must be attached to an existing underwater structure, underwater casting and solidification can be the only effective method of construction.
There are many factors that must be controlled for successful application of cement underwater. Of these, the hardening time, that between mixing and solidification, is particularly important because, if it is too long, the cement does not solidify at all but simply dissolves in the surrounding water, herein the environmental water. The major problem with current cement compositions is the failure to solidify underwater before dissolving.
The temperature at which the hardening reaction occurs is inversely proportional to the hardening time. To raise the reaction temperature and shorten the hardening time, an exothermic substance, such as calcium chloride (CaCl.sub.2), is often combined with base cement and water. The chemical reactions between base cement, water and calcium chloride are as follows: EQU Base Cement.sub.(d) +2H.sub.2 O Base Cement(2H.sub.2 O).sub.(aq) (a) EQU Base Cement(2H.sub.2 O).sub.(aq) 2+CaCl.sub.2 Base Cement.sub.(s) +4HCl+O.sub.2 +Heat (b)
where Base Cement.sub.(d), Base Cement(2H.sub.2 O).sub.(aq) and Base Cement.sub.(s) are the base cement in dry, liquid and solid states, respectively. Heat is the exothermic heat liberated by calcium chloride due to reaction with water.
In underwater applications, the exothermic heat released by calcium chloride must raise the reaction temperature of the liquid cement despite the cooling by the environmental water. Environmental water has significantly higher thermal capacity and conductivity than air and more heat is needed to raise the reaction temperature underwater than in air. A problem with current cement compositions is that exothermic substances do not provide enough heat to overcome the cooling by environmental water, especially cold environmental water.
Current cement compositions fail to provide sufficient heat, in part, because the calcium chloride is in clumps having a small surface-to-volume ratio, a low dissolution rate and a low heating rate. Calcium chloride used in cement compositions is not refined and current processing and packaging procedures do not control water absorption. Calcium chloride is extremely hydrophilic and readily absorbs water from any source including the atmosphere. Water absorption causes the exothermic substance to cake to large clumps. As a result, the calcium chloride is in large clumps several centimeters in diameter. Solid material in liquid dissolves in inverse proportion to the surface-to-volume ratio of the solid. The large calcium chloride clumps have low dissolution rates because of the small surface-to-volume ration. The dissolution rate determines the heating rate of a calcium chloride because calcium chloride only produces heat when it reacts with water. A reduced heating rate results in lower reaction temperatures because the environmental water cools the liquid cement faster than it is heated. Thus, the small surface-to-volume ratio of calcium chloride limits the rate of heat produced.
Another reason current cement compositions do not produce enough heat for underwater applications is that the calcium chloride has reacted with water before mixing with cement. Exothermic heat liberated before mixing depletes the amount of heat produced upon mixing with cement. In some preparations, calcium chloride is dissolved in water before mixing which causes large amounts of heat to be loss before mixing with cement.
Underwater cement applications require that one precisely control the hardening time of the cement. One cannot use cement if the cement solidifies before it can be applied. Moreover, different hardening times are required for different underwater applications. For example, a short hardening time may be required to repair a leak so that the cement solidifies before being carried away in the flowing stream of water. A long hardening time may be needed to plaster a large surface because more time is needed to apply the cement. A shorter hardening time may be needed to repair a vertical surface than on a horizontal surface because of downward gravity flow of the cement on a vertical surface. In addition, the hardening time must allow for transportation time to the underwater site. Further, precise control of the reaction temperature also is needed because the reaction temperature determines the structural properties of the solidified cement such as hardness.
Current cement compositions do not allow one to precisely the hardening time because the heat released from the exothermic substance cannot be precisely controlled. This is because the exothermic agent produces variable amounts of heat. As described above, the surface-to-volume ratio effects the heating rate and the reaction temperature. Current exothermic substances have wide variation in the surface-to-volume ratio and therefore variable amounts of heat are produced. Moreover, current calcium chloride varies on a weight basis because of variable amounts of water content.
Clearly, there is a need for a cement composition that solidifies underwater with a precise hardening time. There is also a need for a cement that the allows for the adjustment of the hardening time to suit the underwater application. To achieve these goals, an exothermic substance is needed that produces sufficient heat to raise the reaction temperature to a point where the cement composition solidifies before dissolving, even in cold environmental water. Further, the exothermic substance must allow one to precisely control the amount of heat produced. The present invention satisfies these needs and provides related advantages as well.